This invention is concerned with stroke control for a press and in particular with methods of and apparatus for controlling the stroke of a press which comprises a stationary platen, a movable platen, and a plurality of columns on which the movable platen is mounted and which move to move the movable platen towards the stationary platen in a stroke of the press.
In the operation of many presses, a workpiece is placed on the stationary platen of the press and a stroke of the press takes place to bring the movable platen into pressing engagement with the workpiece. In such presses, it is necessary to terminate the stroke of the press once the workpiece has been pressed sufficiently and before damage occurs to the press. To cause the stroke to be terminated at the correct time, presses are provided with stroke control apparatus which senses the position of a portion of the movable platen and causes the stroke to be terminated when that portion reaches a preselected point. The stroke control apparatus is set for a particular type of workpiece and then in many cases operates many times without further setting. Consistent results can be achieved provided that the workpiece is always located in the same position on the stationary platen but this may not be possible or desirable. For example, where a sheet material is spread on the stationary platen and is press-cut by having a shaped knife pressed through it, the knife is generally moved along the stationary platen between strokes of the press. Furthermore, positioning of workpieces continually at the same position causes the platens to become worn more rapidly than if the workpieces are positioned at different positions.
Once the stroke control apparatus of a press has been set with the workpiece in a particular position, its accuracy will be reduced if the workpiece is positioned elsewhere. Generally, if the workpiece is positioned near an edge of the stationary platen, once the movable platen contacts the workpiece there is a tendency for the movable platen to tilt thereby affecting the accuracy of the stroke control apparatus. Since stroke control apparatuses sense the position of an edge portion of the movable platen and are generally set with a workpiece in the center of the stationary platen, if the workpiece is near the portion sensed, the stroke control apparatus may react later than is desirable whereas, if the workpiece is remote from the portion sensed, the stroke control apparatus may react sooner than is desirable. Although many presses have balancing mechanisms that alleviate this problem by endeavouring to maintain the two platens parallel to one another, they do not react sufficiently rapidly to overcome the problem. The problem could be reduced by sensing the movement of a central portion of the movable platen but to position stroke control apparatus in such a position is highly inconvenient.